The invention relates to an antenna for transmitting and receiving a radio frequency while mounted on a window of a vehicle. The basic construction of the antenna is of the well known Marconi type consisting of a vertical radiator and an essentially horizontal grounding element at one end of the radiator, but apart of it, whereby said vertical radiator and grounding element are connected to a radio apparatus with separate connecting lines.
This type of antenna is widely used in transmission applications between a base station and a mobile vehicle, due to the benefits involved in the antenna of the type in question. One of these applications is for mobile telephones mounted on vehicles.
One important parameter of a mobile telephone antenna intended to be used in a vehicle is its radiation pattern. Radiation should be concentrated in the horizontal direction as much as possible because the base station is generally at the horizon. Moreover, the radiation pattern in the horizontal plane should be a wide one and symmetrical in relation to the vertical plane in order for the antenna to function properly regardless of the angle formed between the vehicle's direction of travel vector and the directional vector between the vehicle and the base station.
A radiation pattern of the type described above can be achieved by using an antenna of the mentioned Marconi type mounted in the middle of the roof of the vehicle, whereby the roof forms the grounding element of the antenna. This application results in good directional radiation patterns in the horizontal plane, with a minor disadvantage, that the maximum radiation is directed slightly too pronounced upwards from the horizontal plane.
One major problem associated with the telephone antennas fastened onto the external body parts of motor vehicles is the requirement for bored holes. Solutions to this problem have been presented; including the use of suction cups, magnets, etc., in attaching the antenna to the external body parts of motor vehicles. These are, however, rather clumsy solutions and the antenna cable must first pass along the roof of the motor vehicle and from there, by way of the door seal, into the cabin of the vehicle and then to the mobile telephone.
Another prior art solution consists of fastening the antenna onto the outside of the motor vehicle's window and fastening a counterpart onto the inside of the window and the signal is transmitted through the glass.
In addition, U.S. Pat. No. 4,746,925 describes a mobile telephone antenna for mounting onto the window of a motor vehicle composed of two dipole antenna elements. One end of either of the two dipole antenna elements is connected to the antenna cable's central conductor by means of its central electricity conductor and the other dipole antenna element's end is connected to the antenna cable's sheath by means of a first electricity conductor serving as an electrical projector installed adjacent to the central electricity conductor. Additionally, there is, on the other side of the central electricity conductor, a second electricity conductor connected to the antenna cable's sheath also acting as an electrical protector. Additionally, transformer elements protrude from the first and second electrical protectors. An antenna of this type possesses a typical dipole directional pattern; that is to say, a sphere, which means that considerable amounts of energy are directed upwards and downwards in addition to the horizontal plane. The effective radiation area of this type of an antenna remains relatively small in relation to the overall space required by it. The dipole elements form the radiation space while the rest of the antenna is unnecessary space in this respect. The part of the antenna between the dipole elements and the antenna cable forms a balun by means of which the antenna is tuned to a single frequency. Moreover, the structure in question is relatively complicated.
There is thus still a need for an antenna for telecommunications between vehicles and base stations, which antenna would provide the beneficial radiation properties of the Marconi type antenna, and the feasibility of the window mounted antenna of the U.S. Pat. No. 4,746,925. One reason for the absence of an antenna construction having the aforementioned benefits might be the fact that, as the inventor has noted, when an antenna of Marconi type is fabricated according to the conductive foil technique described in the named U.S. Pat. No. 4,746,925, the resulting antenna is very to difficult to construct to match with the remaining parts of the radio apparatus, especially with the standardized antenna cable properties. The main reason for this incompatibly seems to be the high impedance of the antenna, which is in the order of 75 ohms, as compared with the standard 50 ohm cable impedance. This fact causes a discontinuity problem between the antenna and the cable connected to it. The discontinuity causes wave reflection and leads to a decisive weakening of the communication achievable with the radio apparatus connected to an antenna of the tested type.
The antenna as such has proven to have a very good radiation pattern of a wide, flattened torus type, which is even more flattened that of an antenna of Marconi type mounted on a vehicle roof.
The discontinuity problem has now been solved by the present invention.
The basic construction of the antenna needs to be dimensioned carefully taking into consideration the frequency to be transmitted and received by the antenna, as well as the environment where the antenna is to be working. The most important environmental feature is the dielectric properties of the window glass on which the antenna is intended to be mounted. The dielectric properties, i.e. the velocity factor of the glass is the main feature, in addition to the transmission frequency, affecting the dimensions of the antenna.